feat(ui): Erudit blank tiles carry the star (✻) mark

The Erudit variant's blank is the "звёздочка", so render it with a star.
An empty rack blank (and its drag ghost) shows ✻ centred; a placed blank
keeps its designated letter and carries ✻ where the (absent) point value
sits — on the board and in the Stats best-move tiles. The Scrabble variants
are unchanged. Gated by usesStarBlank() in lib/variants.ts.

ui/src/game/Rack.svelte

@@ -3,6 +3,7 @@
   import { BLANK } from '../lib/placement';
   import { valueForLetter } from '../lib/alphabet';
   import type { Variant } from '../lib/model';
+  import { usesStarBlank, BLANK_STAR } from '../lib/variants';
 
   let {
     slots,
@@ -66,8 +67,12 @@
       animate:hop={shuffling}
       onpointerdown={(e) => ondown(e, slot.index)}
     >
-      <span class="letter">{slot.letter === BLANK ? '' : slot.letter}</span>
-      {#if slot.letter !== BLANK}<span class="val">{valueForLetter(variant, slot.letter)}</span>{/if}
+      {#if slot.letter === BLANK}
+        {#if usesStarBlank(variant)}<span class="star">{BLANK_STAR}</span>{/if}
+      {:else}
+        <span class="letter">{slot.letter}</span>
+        <span class="val">{valueForLetter(variant, slot.letter)}</span>
+      {/if}
     </button>
   {/each}
 </div>
@@ -133,4 +138,12 @@
     font-size: 0.7rem;
     font-weight: 600;
   }
+  /* Erudit's blank ("звёздочка") shows its star centred on the otherwise empty tile face. */
+  .star {
+    position: absolute;
+    inset: 0;
+    display: grid;
+    place-items: center;
+    font-size: 1.5rem;
+  }
 </style>